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Solar Gain in Passive Solar Homes

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As with all forms of solar power – solar thermal, PV panels, solar chimneys and others – the amount and quality of sunlight to which a home is exposed is extremely important in holistic home design. Thankfully, again in common with many other forms of solar power, even areas that might appear at first sight to be far too gloomy can, in fact, provide enough energy to reduce heating bills by 50% or more.

The specifics of solar radiation are covered in great detail elsewhere on our site, so we'll just go over the important factors to consider when you survey a building site or existing home for passive solar inclusions.

The prime hours for solar radiation are between 9:00 a.m. and 3:00 p.m. While there may still be solar energy available for collection outside those hours, it is vital that whatever systems you install are fully active and unobstructed during this period of the day.

The solar window must always be calculated so that any obstructions are taken into account or removed.

The strongest sunlight comes from true south. The east and west faces of a building will get early and late sunlight (respectively) but will generate comparatively little heat and power.

Trees on the north face of the building cause no problems with solar systems. Those sited on the east and west may block light but can reduce the heating effect of the low summer sun, which is still hot even at the start and end of the day.

The summer sun tracks higher in the sky than the winter sun, so eaves and overhangs can be used to shield against gathering too much heat in summer, while leaving passive systems open to suck up as much sunlight as possible in winter (see diagram).

Deciduous trees can be useful to block the powerful summer sun and allow the winter sun's energy to hit the building. However, not all deciduous trees lose their leaves quickly – oak trees, for example, retain their leaves well into autumn, which can seriously affect solar gain. Choose carefully and consider keeping the lower branches trimmed back to further reduce losses.

To put things simply, your survey should ensure that the southern face of the building is kept as unobstructed as possible during the heating period of the year. Whether that southern face is one wall or two (at angles) is not as important as making sure it gets uninterrupted, unobstructed radiation from the low winter sun for as much of the day as possible – but especially in the prime hours.

If you can manage that, handling the effects of the hot, high summer sun can be more of an afterthought and relatively easy to handle with careful planning and placement of trees, fences and other obstructions.

Check wind flow, too

While you're doing your solar survey, it's worth checking wind flow around the site as well. In colder climates, a strong wind can carry off an enormous amount of valuable heat, so you will want to shield your home.